And, he added, Mr Cameron "feels powerless - and I think this is a fair conclusion - to go out and start negotiating a thing that he doesn't believe in and he has no idea how he would do it.

"And by the way, nor do most of the people who voted to do it," Mr Kerry said, apparently referring to "Leave" campaigners such as former London mayor Boris Johnson, now the frontrunner to replace Mr Cameron as premier.

Asked by the panel moderator if this meant the Brexit decision could be "walked back" and if so how, Mr Kerry said: "I think there are a number of ways." "I don't as secretary of state want to throw them out today. I think that would be a mistake. But there are a number of ways," he said.

Washington has long supported a strong role for its British ally in the European project, and was dismayed when British voters chose last week to quit the Union.

Now, US officials are calling for a calm debate on Brexit leading to a deal that would allow a close association between London and Brussels to continue.

Some EU leaders, however, insist that Mr Cameron must move quickly to invoke Article 50 and begin divorce talks, to put an end to political and economic uncertainty.